The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system with a catalytic converter comprising a three-way catalyst.
Such a control system is a feedback control system, in which the system comprising an O.sub.2 sensor for detecting the concentration of oxygen in the exhaust gases, an air-fuel mixture supply unit, an on-off type electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply unit, and an electronic control circuit. The electronic control circuit comprises a comparator for comparing the output signal of the O.sub.2 sensor with a predetermined value, an integrating circuit connected to the comparator for integrating the output of the comparator, and a driving circuit connected to the integrating circuit for producing driving pulses for driving the on-off type electromagnetic valves. The O.sub.2 sensor generates an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture induced in the engine cylinder.
The output voltage of the O.sub.2 sensor is higher than a predetermined voltage when the oxygen concentration of the exhaust gases is smaller than a predetermined ratio corresponding to the stoichiometric air-fuel ratio in the air-fuel mixture for the combustion of the mixture and is lower than the predetermined voltage when the oxygen concentration is greater than the predetermined ratio. The duty ratio of the driving pulse varies in dependency on the output of the integrating circuit to control the air-fuel ratio of the mixture to be supplied to the cylinder to the stoichiometric air-fuel ratio.
In order to control exactly the air-fuel ratio, it is desirable to provide detecting means which may detect the engine operation in the idling operation, the steady state operation and the wide open throttle condition and to provide a control circuit for controlling the circuit constant of the feedback control circuit in accordance with the engine operation.